Collagen is a potentially useful biomaterial owing to its formation as a natural polymer, its ready availability, its biocompatibility and its inherent strength. As a result, collagen-based matrices are widely used, especially in clinical applications requiring the use of a relatively stable, biodegradable polymer. Such matrices are particularly employed as implants in a variety of surgical and medical procedures. They are generally prepared for this use by purifying native collagen and cross-linking the purified fibers to increase the mechanical strength of the matrix and improve resistance to enzymatic degradation.
When collagen is obtained from animals, usually in the form of hides or bones, various contaminating tissue macromolecules and blood must be removed and the collagen purified in order to be reused. Collagen normally has to be ground, shredded or pulverized, washed extensively with various solvents and reformed into various materials. Collagen particles and fibers so produced must be recross-linked in order to regain the strength and physical properties which allow it to be used as a biomaterial. Agents used for cross-linking collagen have been described for centuries in terms of its being tanned for use as leather. Various methods of cross-linking collagen have been used for this purpose; however, most methods used for cross-linking collagen, including some methods that allow collagen to be cross-linking as bioimplantable devices, have proven to be toxic for cell growth, and/or not biocompatible as implants.
Until recently, the cross-linking agents of choice have been glutaraldehyde and related non-physiological agents, which react with amino acid residues in the collagen molecule to cross-link polypeptide chains. The use of such cross-linking agents to prepare collagen matrices for clinical or veterinary applications is currently in disfavor, however, owing to the incidence of toxic and immunogenic reactions attributed to leaching of these agents from the matrix into the surrounding body areas of the host. Localized inflammation and more complex systemic reactions are particularly common side-effects of collagen-based implants cross-linked with glutaraldehyde. Further, such cross-linking agents tend to unfavorably alter the collagen amino acids side chains that reduces cell attachment resulting in a matrix that does not promote cell ingrowth from the host. Such cross-linked materials are susceptible to being either walled off or, promoting foreign body reactions.
Accordingly, other, more physiologically-acceptable, cross-linking agents have been proposed for stabilizing collagen matrices, such as the carbodiimide or succinimidyl active ester agents described in U.S. Pat. No. 4,703,108. The cross-linking process described in this patent, however, requires a relatively harsh dehydrathermal step, and precludes the incorporation of heat-sensitive components into the collagen matrix in conjunction with the cross-linking reaction.